Everyone knows disaster movies are totally unrealistic — massive climate change doesn't just happen in a few months, right? Wrong. Some Canadian scientists have figured out that it did once, and very easily could again.

Earth's climate has shifted drastically many times in its history, but barring massive asteroid impacts, climate change tends to play out over thousands or tens of thousands of years. Plenty of time to pack away the summer clothes and buy a nice warm coat when you spot an ice age coming. But researchers at the University of Saskatchewan recently discovered that some ice ages come on quite rapidly.

By taking some very thin slices of a mud core sampled from a really old lake (Ireland's Lough Monreach), the Saskatchewan team got a high-resolution look at varying oxygen and carbon isotope levels in the lake's history. The analysis revealed events happening month by month in the lake's ancient past. They found that things got very cold very quickly during the "Big Freeze" (more scientifically referred to as the Younger Dryas), a small ice age that occurred about 13,000 years ago. How quickly? The lake basically froze solid within a few years, and it might even have happened within a few months.

The Younger Dryas is thought to have been caused by the sudden emptying of Lake Aggasiz, a massive freshwater sea that covered a much of what is now mid-Canada and the northern U.S. At some point, the contents of the lake poured down through the Great Lakes and down the St. Lawrence River, flooding the Arctic Ocean and the North Atlantic with a deluge of fresh water. Not only did this raise global sea levels, it severely disrupted ocean currents in the area, scrambling the climate and causing an instant ice age. It took about 200 years for things to get back to normal.

Since there's no Lake Aggasiz around to empty itself and cause an ice age, we're off the hook, right? Wrong. The Greenland Ice Sheet (which is an enormous slab of ice that covers most of Greenland) has been melting at a rate of about 50 cubic miles of ice per year. Because the ice sheet basically keeps itself cold, scientists worry that the growing melt zones each summer will lead to a runaway meltdown. If the sheet were to melt away suddenly, it could very well lead to a disruption of ocean currents similar to the one preceeding the Younger Dryas.

Of course, that would probably be the least of our problems, since sea level would rise more than 20 feet. And you still can't outrun cold.